To amend a risk assessment in order to make changes just head to the completed forms section for the relevant assessment be it 

General
COSHH
Fire
Manual
DSE

You will now see a list of all your completed assessments and at the end of each  will see a little pencil icon next to the title heading named view / edit icon. Click on the pencil and the form will open.

Make your changes in the same way you did when you first created the form, at the bottom of the form there is a save form button.

To down the completed form, once you have it open just click download form (it's at the top right of the form)